Schmidt television projector with spherical aberration corrector



Dec. 13, 1949 J. G. BAKER ETAL 2,491,072

SCHMIDT TELEVISION PROJECTOR WITH SPHERICAL ABERRATION CORRECTOR Filed June 19, 1945 2 Sheets-Sheet 1 ATTORN EY5 Dec. 13, 1949 J. G. BAKER ETAL 2,491,072

SCHMIDT, TELEVISION PROJECTOR WITH SPHERICAL ABERRATION CORRECTOR Filed June 19, 1945 2 Sheets-Sheet 2 ATTORNEYS Patented Dec. 1 3,, 1949 2,491,072

UNITED STATES PATENT OFFICE James Baker, Waban, Mass, and Constantin S. Szegho, Chicago, Ill., assignors of one-half to The Perkin-Elmer Corporation, Glenbrook, Conn, a corporation of New York, and one-half to The Raulan'd Corporation, Chicago, 111., a eorpora-tion'of Illinois Application .J-u-ne 19, 1945, Serial No.600-,257

13 Claims. (Cl.1787.5)

This invention relates totelevision receivers emditi'on-al light by vigne'tting and this effect is enp oy g a cathode ray tubea ideonc hanced by the ,iormation of the central opening particularly with anew television receiver which in the correcting plate and mounting .the tube to includes an optical system for the purpose of :prO- extend'throug-h the opening. Because of the facjecting upona viewingscreenan enlarged image 5 tors mentioned, the priorsystem in a commercial of that appearing on the fluorescent screen of the form is constructed to operate at a speed of about tube. The optical system of the television r8- F/OIM, equivalent to an effective speed of about ceiver of the invention, whileoftheschmidt type, F/018'5, and with s. held of about 45 degrees. ofiers numerous advantages over an ordi a y The present invention is directed to the Pro- Sohmidt System in that it maybe constructed t 10 vision of .a novel television receiver containing permit an increase in speed or aperture without an optical system, which, at considerable field loss of definitionor to give better definition at the angles, gives markedly superior definition to that same speed .or aperture. .Also, in the .new i eof an ordinary Schmidt system of the .same E- :c iver, elem n s f he p l y e y b number, or, for the same definition as that given em ia ed in he ca ode ray tube n elope i by an ordinary Schmidt system, permits an in:- such manner that there is less loss. orl'ight. crease in speed or aperture. The optical system The use of optical systems :in television receivof the new receiver may be constructed to operate ers, employing a cathode ray tube,..-for-t'he purpose at a speed of F/O.6'0 and, at that speed, gives conof projecting upon a-viewing screen an enlarged siderably better definition than the prior system,

image of that appearingon' the fiuorescentscreen above referred to, operating at F/0;74. In addiof the tube is not new, and systems of various tion, in television use, the light lost in the new kinds utilizing lenses or spherical mirrors have system because of silhoue'tting and vignetting is been proposed for the purpose. The combination substantially less than the loss in the prior sysof a spherical mirror and an aspheric correctingv tem.

plate counteracting the spherical aberration of The optical system of the newreceiver includes the mirror, which was devised by Schmidt, is an a spherical mirror, a. pair of thin shells with con- .optical system that is obviously desirable for-telecentric radii and concentric on either side of the vision purposes, because the Schmidtsystemhas center of curvature of the mirror, and a correctlong been known to afford clarity of definition at ing plate. Each of the shells adds a p p y speeds, and fields of view .at such speeds, that chosen amount of negative spherical aberration are not obtainable with refractingsystems. Howto the system with a resultant drastic reduction ever, the previous applications of the .S'chmidt of the positive spherical aberration of the mirror. system to television projection use, with which I The mirror and the two concentric shells form a. am familiar, .are open to .2. number of objections perfectly symmetrical system and the spherical and are :also subject .to limitations in performaberration of this system is considerably less than ance which restrict their use. that of the mirror alone. The correcting plate is, In one such prior televisioni'eceiven. the oath-- therefore. weaker or flatter than that required ode ray tube projects through arr-opening in the in an ordinary Schmidtsystemof equalspeed-and correctin plate toward the mirror and the lightthe performance at a given field-angle is correis radiated from therear surface of theiluores- 40 spo'nding'lyimproved;

cent screen to the mirror. .As the fluorescent While the use of two shells as above described screen radiates the major portion otits light in ,gives the best results, -it is possible to obtain part the forward hemisphere, the arrangement deof the advantages of theinvention by employin scribed operates .at .an initial disadvantage. The Y ,.a single shell. In such an arrangement, a heavier interposition of the fluorescentscreen in such-a burden is placed upon the correcting plate .and, system blocks off a large portion of the aperture because of that, the plate must be steeper, so that and, in order that this lost light may not illumidefinition is sacrificed at a .given speed, or, for a mate the fluorescent screen with consequent loss given standard of definition, there is a reduction of contrast in the final imagmit is necessary to in speed or aperture. However, the use of the prevent "such action. The desired result has been single shell with the mirror and correcting plate achieved either by iorming the mirror with a cenproduces a system which is superior to an orditral hole of-the size and shape of the fluorescent nary Schmidt system.

screen or by applying an opaque disc of suitable In the preferred form of the new television resize and shape tothe central area nr'the. mirror. .ceiver, elements re! the optical system are in- The useof such expeulentsresultsin a loss ofad '55 comorated in the tribe to form part; of the tube 111.000". commercially as Pyrex, because of the heat that is envelope. Thus, one of the shells may form the end of the tube adjacent which the fluorescent screen is mounted and the mirror may form part of the wall of the enlargement of the tube within which the screen lies. When the mirror and shell are used in the manner described, they must be made of materials selected to withstand the temperatures to which they will be exposed in the-- processing and operation of the tube.

For a better understanding of the invention, reference may be made to the accompanying drawings, in which 3 mensions as the shell I3, but it is made of the glass known commercially as BSC-2. With the system of the dimensions above given, the picture diameter on the projection screen I8 is 76.772",

- the distance from the-correcting plate to the pro- Fig. 1 is a diagrammatic view of the elements zontal dimensions of the plate being multiplied.

Fig. 4 is a diagrammatic sectional view of the fluorescent screen of the cathode ray tube of Fig. 2; and

Fig. 5 is a longitudinal sectional view showing another form of the new receiver.

One form of the new television receiver, in which elements of the optical system form parts .bf the tube envelope, is illustrated in Fig. 2 and jection screen is 96.86", and the magnification is 23.79 times.

As pointed out above, the purpose of the shells is to introduce a properly chosen amount of negthe dimensions of a specific system suitable for such use are shown in Fig. 1.

The cathode ray tube of Fig. 2 has an envelope formed with a neck H1, in one end of which are mounted the usual cathode serving as a source of electrons, the means for forming the electrons into a beam, the control and focusing electrodes, and the beam-deflecting device. At its other end, the neck opens into an enlargement ll, one

'wall of which is formed by a spherical mirror l2 silvered on its concave surface. The opposite wall of the enlargement is formed by an optical shell l3 which is concentric with the mirror. The edge of the shell is connected to the outer edge of the mirror by a frusto-conical glass section l4 and the neck. mrror, section. and shell form an envelope which is vacuum-tight. A fluorescent screen I5 is mounted within the envelope and has .a convex surface facing the mirror.

A correcting plate I6 is mounted outside the .with its concave surface facing the plate.

The dimensions of the optical system of a typical television receiver of the invention are given in Fig. l, where it will be seen that the mirror has a radius of 8.500 and a clear aperture of The shell I3 is made of the glass known it must withstand, and it has an outer radius of 4.000" and an inner radius of 3.750" so that its thickness is .250. The shell is concentric with the mirror. The correcting plate I6 is of the glass referred to commercially as DBC-l and it has a clear aperture of 6.912". It is figured as needed from a basic radius of 24:9" for each face.

.The fluorescent screen has adiameter of 3" and ative spherical aberration into the system, so that the positive spherical aberration of the mirror is drastically reduced and the work to be performed by the correcting plate is also greatly reduced. It is well known that the nearer the correcting plate in an ordinary Schmidt system is to flatness, the better is the performance at a given fieldangle. As the correcting plate in the optical system of the new receiver is weaker or flatter than that required in an ordinary Schmidt system of equal speed, the definition of the optical system of the new receiver is markedly superior to that of an ordinary Schmidt system of the same F-number at considerable field angles. Conversely, for the same definition in the projected image, the speed or aperture of the system of the receiver of the invention can be increased over that of a conventional Schmidt system. Another advantage in using the shells is that the color correction for most zones is better than in an ordinary Schmidt system, since the positive curve on the correction plate is always acting against the negative contributions of the shells.

The shells may vary in thickness, and as their thickness increases, the burden on the correcting plate is reduced and a flatter plate may be used. The shell thickness adopted represents a compromise between the size of the mirror and the correction burden on the correcting plate. Increasing the thickness of the shells reduces the burden on the plate and increases the mirror size for a given focal length of the system. For a system operating at a speed of F/0.60, the permissible upper limit inthickness of the shells is A; of the equivalent focal length, that is, ,4; of the distance between the center of curvature of the mirror and the focal point. For systems to operate at lower speeds, the thickness of the shells may be greater, as, for example, for a systern to operate at F/ 1, the shells may have a thickness up to /2 the equivalent focal length. With shells of that maximum thickness, no correcting plate is required. However, shells or less thickness are preferred and with shells of a thickness of .25" in the example of the system described, variations in correction over the aperture are minimized. The shells need not be identical in thickness, but if one is thicker than the other, the overall correction by zones is impaired. The radius of the shells should be as long as possible and the maximum radius is, of course, that at which the surface of one shell is in direct contact with the fluorescent screen. The glass used for the shells may have a mean index of refraction (no) varying from about .1

to 1.8 and it is advantageous to employ a glass having as low a dispersion, that is, as high a v-value, as possible. The shells introduce negative color which is corrected bythe plate. By

niakingfi the shells ofiglass of"low-dispersion} linrdeais'imposed' upon' the' plate; The achro matism-bi thesliells-can' lreovercome by causin'gthemto-depart from" concentrici-ty 'with the mirror but; for a given standard of definition; this results iii-a reduction inthefield angle.-

Thefluorescent screen in the new receiver? which corresponds to the-photographic in an ordinary Schmidt camera; is approximately spherical and-concentric with the mirror. The departures from sphericity and" also-fromcon centricityareobviously necessary;- because theprojection screen is at-afinite distance, whereas tlie-Schm-idt camera; the-corresponding dis tance is infinity. The shape and position of-thefiiio'rescent screen will vary; therefore: withthe distance'between-the correctingplate and the proj ectionsc-reen butmay be readily =-determined'2 For' mos't projection'purpo'ses; a screen ofspheri' cal shape-= is suitable but for highest precision; the screen will be-* formed with the slight zone indicated-'bydottedlines in Fig. 4. Thegreatestdepthofthezone will be-0.0035" in a scream of' three inch-diameter: 4

In theforegoing. I have described a "form'- of tlie new television receiver irrwhich elements of th'e optical system are incorporated in the envelope of the *cathode ray' tube of the receiverto form wallsbounding the evacuated space. This form ozf reeeiver is preferred, since the light striking the mirrorcoines'from the-forward zone-of the screen.- Insome instances; however; it maybe desirable to constructthe 'receiverwith the --optic'al 's'ystem wholly independent ofthe-cathode-raytube'and' such an arrangement is-show'n infie; 5'? In-the Fig. 5 form'of the 'new'receiver; thespher icalmirror l9 is provided with a central" open ing or elsethe central area is covered by an op'aqueblackb'afllezti The shells 2!, 22 and the" correctingplate 2 3 are formed with central open ings through which the cathode ray tube-M me? be inserted with the-enlarged end 24a of the tube facing themirror. The fluorescent screen 25 within the end. ofthe.v tube, is. disposed at the proper distancefrom the'mirror by reason of the position. of: the tube; Theglight'striking. theymflror comes from the back surface; Of the; Screen; and passes.throughthewall. of thetube envelope on its way to the mirror; The correcting plate must, accordingly, be designedeto correcttforthe' spherical aberration of the mirror as modified'iby the shells as well as. for the erronintroducedtby the tubeend wall, as will. beobvious. Tlie diameter of: the .baffieis. substantially the sarnaas that? off the. screen. projected in straight-lines parallell to the. axis of the. system.

As pointed out above; the'mat'erials employed for-the shells andcorrecting plate; in the optical system of the vnewreceivershould be selected in order that chromatic aberration may be reduced as much: as possible, but; in some applications;- other requirements must-be fulfilled. Thus; in a'systein, such-as that shown: in Fig. Zji-n -Which the mirror and one shell form parts of the en-r vlelope of. the cathode. ray tube, the mirror-and shall should: be made of a material capabla oi withstanding the temperatures to which theyare exposed in theprocessing operation of the tube. The materials used in the correcting plate tation on the choice of the material for the first shell. The selection of the materials will be clear to one skilled in the art of optical design and second shell will then be chosen-to obtain, the desired chromaticeif'ects; inviewpfthe limi pressed ink-an e'quatlon rel-atlng.=- the. lens power ofthe shel-ls. and correcting plate to their-*- d sired dispersions' for-any speeifim examplepf the system.

We claim:

1. ir'nage projection device" comprising a cathode ray:- tub'e having a fluorescent screen on which-a lightimage isproduced; and arr-axially; alig ned optieal system comprising a spherical re- Hector placed to receive light from the screen onits: concave surface, a spherical shell'concentr-ic with the reflector; the shell adding negative spherical aberration-to the-system toreducethe positive spherical -aberration of the refi'ecton-and anaspher-ic correcting plate lying outside the en'- velope-at the center of 'curvature -of the reflector and shell in-thepathof rays passing from the reflector through' the shell and figured to correct for-the residual spherical aberration of thecom bination oftherefictor and shell:

An image-'- projectiondevice comprisinga cathode ray 'tube having- -a fluorescent 'screen' on which: a light image is -producedgand an axially aligned opticalsystemcomprising a spherical re= fictor forming part of; the tube envelope, the reflectorbeing placed} to=-receive light from the screen on its-concave surface; apair'of spherical shellsconcentri'cwit-h the-reflector andiying outside the tube envelope and with their; concave faces opposed} the shells adding-negative sphericalaberration tothe system to-reducethe positive: spherical aberration of the-reflector,- and an aspher-lc correctingplate lying between the shells and figured-- to correctfor the residual spherical aberration of-"thecombination ofthe reflector and shells."

35 An image projection device comprising a; cathode ray' tube, the-envelop off'which has. an end; Walt-formed ofa shell of spherical curvature with its concave surface; facingoutwardly and with a curved fluorescent screen mounted; inside the envelope adjacent the convex surface of'said end"wall; and anpptical system aligned with the screen; and comprising a sphericalfreflector concentric with Said; endwall and receiving light from the screen. the tube end wall. adding negative-spherical ab'errationto the system to reduce. the positive sphericaraberration. of the refi'ecton'. andan a's'pheric correcting; plate lying. outside the tube in the pathof'rays passing from. the reflector-through the shell and figured to correct forthe'residual; spherical aberration of the combination-10f therefitector and the tube end wall;

t." An image projection device comprising, a cathoderay tube; the; envelope oi which has an end wallof'sphericat curvature with its concavev surface-facing outwardly with a curved, fluorescent'screemmountedinside the envelope-adjacent; he; o xq u fa ez f a dg nd Wall; d rip a 8 $23 1 ija i pfidi th h 5. 13 1. E SF- in s palrefie tor.c centr cw th. an wa i a i. Qrmingnart oi. h. n op th fle r ceivins lieht fr m he. cr en. a spherical} shellrconw ntric-ou side he u e with-the. lectee aod r ns bero id-thecenter of. curvature withitsconpave face opposed'to: thatiota the; end wall; the; shell: anctend wall. adding: mega-1 tiie' sphericall'.aberration to the 'system to. reduce th sitivegsphericaliaberration. of the reflector;

andranrasphenie correcting platelying outside thetube betweenthe end wall and shell and figured to correct for the residual spherical aberration of the combination of the reflector, the end wall,

and the color removal conditions can be exand the shell.

amma

- 5. An image projection device comprising a cathode ray tube and an aligned optical system, the tube having an envelope, including a neck and an enlargement at the end of the neck, and a fluorescent screen within the enlargement, and the system including a spherical shell forming the end Wall of the envelope enlargement and a spherical reflector forming the wall of the enlargement surrounding the neck, the shell and reflector being concentric about a center of curvature outside the envelope and the reflector receiving light from the screen and reflecting it through the shell, the shell adding negative spherical aberration to the system to reduce the positive spherical aberration of the reflector, and an aspheric correcting plate lying outside the envelope in the path of rays passing from the reflector through the shell and figured to correct for the residual spherical aberration of the combination of the shell and the reflector.

6. An image projection device comprising a cathode ray tube and an aligned optical system, the tube having an envelope, including a neck and an enlargement at the end of the neck, and a fluorescent screen within the enlargement, and the system including a spherical shell forming the end Wall of the envelope enlargement and a spherical reflector forming the Wall of the enlargement surrounding the neck, the shell and reflector being concentric about a center of curvature outside the envelope and the reflector receiving light from the screen and reflecting it through the shell, a second shell concentric with the first and lying at the opposite side of said center of curvature, the shells having their concave surfaces opposed and adding negative spherical aberration to the system to reduce the positive spherical aberration of the reflector, and an aspheric correcting plate lying between the shells and figured to correct for the residual spherical aberration of the combination of the shells and the reflector.

7. An image projection device as defined in claim 5, in which the correcting plate lies at the center of curvature of the shell and reflector.

8. An image projection device as defined in claim 5, in which the fluorescent screen is aligned with the tube neck and the opening in the reflector is not substantially smaller in area than the area of the screen projected by parallel lines. I

9. In an image projection device including a cathode ray tube having a fluorescent screen adjacent the inner surface of its end wall, the combination of a spherical reflector receiving light from the screen, a spherical shell concentric with 10. In an image projection device including a cathode ray tube having a fluorescent screen adjacent the inner surface of its end wall, the combination of a spherical reflector receiving light from the screen, a pair of spherical shells concentric with the reflector and lying on opposite sides of the center of curvature thereof withtheir concave faces opposed, at least one of the shells lying outside the tube, and an aspheric correcting plate lying outside the tube at the center of curvature of the reflector and shells and figured to correct for the residual spherical aberration of the combination of the shells and reflector and for the error introduced by the thickness of the end wall of the tube.

11. An image projection device as defined in claim 9, in which the tube extends through aligned openings in the shell and plate and the reflector has a central non-reflecting area aligned with the screen and of an area not substantially less than that of the screen projected by parallel lines.

12. In an image projection system, a cathode ray tube having an envelope comprising a neck and an enlargement connected thereto, the enlargement having an inwardly convex end wall facing the neck, a spherical reflector forming the Wall of the enlargement surrounding the neck and facing the end wall, said end wall being transparent and having inner and outer spherical surfaces which are substantially concentric with one another and with the reflector, and a screen on the inside of the end wall facing the neck and reflector.

13. An image projection device comprising a cathode ray tube having a fluorescent screen on which a light image is produced, and an axially aligned optical system comprising a spherical reflector placed to receive light from the screen on its concave surface, a pair of spherical shells concentric with the reflector and lying on opposite sides of the center of curvature thereof with their concave faces opposed, at least one of the shells lying outside the tube, and an aspheric correcting plate lying outside the tube between the shells and figured to correct for the residual spherical aberration of the combination of the shells and reflector.

JAMES G. BAKER. CONSTANTIN S. SZEGHO.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,170,979 Straubel Aug. 29, 1939 2,229,302 Martin et al Jan. 21, 1941 2,273,801 Landis Feb. 17, 1942 2,295,779 Epstein Sept. 15, 1942 2,298,808 Ramberg -2 Oct. 13, 1942 2,305,855 Epstein Dec. 22, 1942 2,344,756 Warmisham Mar. 21, 1944 2,440,735 Cawein May 4, 1948 FOREIGN PATENTS Number Country Date 883,937 France Apr. 5, 1943 477,406 Great Britain Dec. v28, 1937 487,241 Great Britain June 16, 1938 541,650 Great Britain Dec. 5, 1941 544,694 Great Britain -s Apr. 23, 1942 557,771 Great Britain Dec. 3, 1943 OTHER REFERENCES Maksutov, Journ. Opt. Soc. Amer. vol. 34, No. 5, May 1944, pp. 270-284. 

